1. Field of the Invention
The present invention relates to a semiconductor integrated circuit, and more particularly relates to a frequency compensation of an oscillator contained in a semiconductor integrated circuit.
2. Description of Related Art
In association with the progress of the information processing technology, the semiconductor integrated circuit having high precision has been required. The semiconductor integrated circuit has an oscillator for providing the clock signal. Correspondingly to the higher precision of the semiconductor integrated circuit in recent years, the oscillator built in a semiconductor integrated circuit is required to output the clock of the higher precision. For example, there is a case that the oscillator which outputs the clock of 8 MHz±2% in the assurance temperature range of the semiconductor integrated circuit is required. In order to design the oscillator for outputting the clock of the high precision, it is necessary to consider the temperature dependency. With regard to the oscillator, a technique for carrying out the compensation corresponding to the temperature (hereafter, referred to as a trimming) is known (for example, refer to Japanese Laid Open Patent Application JP-A-Heisei, 1-93904).
FIG. 1 is a block diagram showing the configuration of the temperature compensation type voltage control piezoelectric oscillator noted in the above mentioned Japanese Laid Open Patent Application JP-A-Heisei, 1-93904. With reference to FIG. 1, the environment temperature is detected by a temperature detector 101. The analog output of a temperature detector 101 is supplied to an analog-digital converter 102. The conversion output is sent to a memory circuit 103. The memory circuit 103 stores in advance a temperature compensation code corresponding to the conversion output of the analog-digital converter 102 that is a temperature address signal, in order to compensate the temperature dependency of the frequency of an oscillating element 118 used in a voltage control piezoelectric oscillator 105. Here, when the environment temperature is changed, the temperature address signal is also changed, and the temperature compensation code corresponding to the changed temperature is read from the memory circuit 103.
The read temperature compensation code is converted into an analog signal by a digital-analog converter 104, and sent through a terminal 110 and a resistor 107 to the cathode side of a variable capacitance diode 106 included in the voltage control piezoelectric oscillator 105. The control voltage to change the oscillator frequency is applied to the anode side of the variable capacitance diode 106 via a terminal 111 and a resistor 108. A resistor 109 has a high resistance of 100 kΩ or more, similarly to the resistors 107, 108 and holds the anode side of the variable capacitance diode 106 at the zero potential when the control voltage is not applied.
The analog-digital converter 102 is a device for converting the analog signal outputted from the temperature detector 101 to a digital signal. The analog-digital converter 102 defines a reference voltage supplied from a reference voltage supply terminal (not shown) as a reference voltage and converts an analog signal into a digital signal.
When the environment temperature is changed, the temperature compensation code changed correspondingly to the change is read from the memory circuit 103. Thus, the output voltage of the digital-analog converter 104 is changed correspondingly to the temperature change. As for the output from the digital-analog converter 104, the voltage to compensate the temperature dependency of the frequency of the voltage control piezoelectric oscillator 105 is applied to the cathode side of the variable capacitance diode 106. The capacitance of the variable capacitance diode 106 is controlled so as to be changed correspondingly to the output of the digital-analog converter 104 so that the oscillation frequency of the voltage control piezoelectric oscillator 105 is consequently constant.
FIGS. 2A, 2D are graphs showing the temperature dependency of the frequency and the variable frequency characteristic without temperature compensation of the frequency. FIG. 2A shows the temperature dependency of the frequency without temperature compensation, and FIG. 2B shows the variable frequency characteristic without the temperature compensation, respectively.
FIGS. 3A, 3B are graphs showing the temperature dependency of the frequency and variable frequency characteristic with the frequency temperature compensation. FIG. 3A shows the temperature dependency of the frequency with the frequency temperature compensation, and FIG. 3B shows the variable frequency characteristic with the frequency temperature compensation, respectively. Comparing FIGS. 2A, 2B and FIGS. 3A, 3B, the deviation of the frequency in the change of temperature is suppressed in the temperature compensation type voltage control piezoelectric oscillator according to the referenced document by carrying out the frequency temperature compensation. For example, with reference to FIG. 3B, even if the environment temperature is changed from 25° centigrade to Tl° centigrade, the change in the central frequency of the variable frequency characteristic is small.
Thus, according to the technique noted in the Japanese Laid Open Patent Application JP-A-Heisei, 1-93904, digital values to be used in the temperature compensation corresponding to the input digital values to the memory circuit 103 under the various temperatures are stored in advance in the memory circuit 103. Hence, the oscillation frequency is stabilized by a simple method without any complicated computer calculation.